The invention relates to a process and an apparatus for biological wastewater purification.
For biological wastewater purification in sewage treatment plants, the activated sludge process is generally used. In this process, different types of constituents in water are used for respiration or are partially utilized for the synthesis of new biomass by microorganisms (bacteria) suspended in the wastewater in an activation tank in a first process stage, the so-called activation. In the course of this, nitrate is converted to molecular nitrogen (denitrification) and CO.sub.2 is formed. A large part of the gases is removed by ventilation and mechanical circulation and leaves the system via the gas phase. In a second process stage, the so-called secondary clarification, the bacteria are then separated from the water in a secondary clarification tank by sedimentation and can then be transported back to the activation tank. Sewage treatment plants and processes of this type are described, for example, in "Abwassertechnik" [Wastewater engineering], Hosang and Bischof, 9th edition, Verlag B. G. Teubner, 1989.
Owing to increased stringency of legal requirements of wastewater purification and possibly owing to an increase in the pollutant loading of the wastewater, an expansion of existing sewage treatment plants is frequently necessary in order to improve their purification capacity. Generally, the tank capacities need to be increased for this. In the case of the secondary clarification tanks, the surface area generally needs to be increased, whereas in the case of the activation tanks, the capacity can also be increased by increasing the depth of the tank while maintaining a constant surface area, which decreases the space requirement and the construction costs.
However, it has been found that, in sewage treatment plants, as the depth of the activation tank increases, the problem of sludge flotation in the secondary clarification tank occurs to an increasing extent. This gives rise to relatively serious problems in operating the sewage treatment plant and relatively high expenditure is necessary to cope with sludge flotation of this type.
A constant or periodic occurrence of sludge flotation can lead to unwanted discharge of sludge from the sewage treatment plant, which can cause the permissible limit values, for example for chemical oxygen demand (COD), biochemical oxygen demand (BOD.sub.5), nitrogen and suspended matter, to be exceeded. In addition, on account of the loss of activated sludge, the bacterial mass decreases, which reduces the purification capacity of the sewage treatment plant.
Since hydrostatic pressure increases with increasing depth of the activation tank, a higher proportion of gas is dissolved in the mixed liquor, which leads to sludge flotation owing to formation of gas bubbles in the activated sludge in the less deep secondary clarification tank. In addition, excessive oxygen input into the denitrification stage of the activation tank adversely affects the denitrification. As a result, uncontrolled denitrification can take place in the secondary clarification tank and cause a sludge flotation.
To solve the problem of excessive sludge flotation in sewage treatment plants, attempts were made to control the oxygen content better or to prevent the input of atmospheric oxygen for the denitrification stage of the activation tank. It has also been proposed, instead of a conventional secondary clarification, to provide a dissolved air flotation, or to use an additional degasification zone or a flat degasification tank with aeration by coarse or medium bubbles. Furthermore, the use of a weir having a special overflow edge, a weir cascade and, if appropriate, an additional submerged wall with a possibility for taking off foam have been described (Korrespondenz Abwasser, Volume 43, No. 6, June 1996, pages 1083-1086, Report of the ATV Specialist Committee 2.6: "Aerobe biologische Abwasserreinigungsverfahren" [Aerobic biological wastewater purification processes]).
However, the measures mentioned above either increase space requirements of the activation stage and are cost-intensive in construction and operation, or they only partially prevent flotation.
DE 43 29 239 A1, furthermore, has disclosed a process and an apparatus for biological wastewater purification, in which process and apparatus, with feed of air or oxygen, constituents in the water are used in respiration by the bacteria, the bacterial content in the activation tank being increased by lamellae packs arranged to slope upward, and the activated sludge being thickened and the lamellae packs being arranged at the end of the flowpath of the activation tank at a distance upstream of the outlet. This process is said to decrease markedly the capacities of the activation tanks, a width of 5 m, a length of 10 m and a depth of 4 m being mentioned as exemplary activation tank dimensions. This process and this apparatus are not concerned with deep activation tanks or problems of sludge flotation.